1. Field of the Invention
This invention relates to infrared detectors and, more specifically, to forming infrared detectors having semiconductor films of different composition in a mosaic pattern in the same focal plane.
2. Brief Description of the Prior Art
Infrared detectors of the type having mercury cadmium telluride film for detecting infrared radiations normally have a very small frequency window of operation, this window being based upon the percentage of mercury to cadmium in the film. These windows have been very accurately determined and it is also known that the change in frequency response is a linear function of the change in ratio of mercury to cadmium in the mercury cadmium telluride film. Mercury cadmium telluride detectors, when formed with a single layer of mercury cadmium telluride, are therefore responsive to a predetermined infrared frequency window, depending upon the formulation of the film. While such infrared detectors find great utility in the art, it is often desirable or even necessary to provide detectors which have a greater window of operation than is possible with use of a single mercury cadmium telluride layer of a single composition. In order to increase the frequency range of such detectors, it has been determined that more than one different mercury cadmium telluride composition, each with a different ratio of mercury to cadmium, can be used to provide response to additional windows of infrared frequencies. Hg.sub.(1-x) Cd.sub.x Te films for different values of x which correspond to material with different wave length cut-offs have been prepared by successive deposition of one layer on top of another. These films have been deposited by liquid phase epitaxy (LPE), metallic organochemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) methods with MBE being the one most used. Detectors of this type have been provided in the prior art by forming a single layer of mercury cadmium telluride over a cadmium telluride substrate such as by liquid phase epitaxial growth and then forming a second mercury cadmium telluride layer of a different composition over the first mercury cadmium telluride layer, also by liquid phase epitaxial growth, and then etching away a portion of the topmost layer to expose regions of the two layers to the incoming infrared radiations. Such detectors are responsive to two different infrared frequency windows, thereby increasing the bandwidth of the detector. Problems with such prior art methods are that the two different mercury cadmium telluride layers of different composition are not in the same focal plane. Furthermore, the etch back of the first layer of film in the two layer system results in poor image definition. Accordingly, it is desirable to provide an infrared frequency detector which has a large window of operation and yet does not display the problems inherent in the prior art system described above.